yup...90-91 are the "H" models.
Pro Circuit and FMF are listing them discontinued, but it could be worth calling both of them to see if there's anything they could do...just don't get your hopes too high.
You could try PSI and DG also...but if the other two biggest names in exhaust aren't making any, it'll probably be another dead-end.
There are small/private guys out there that make pipes too, specifically for guys such as yourself that have an older bike and can't get one from anywhere else...take a look here...it's an older thread, so not sure how reliable some of these sources are...
http://www.kxriders.com/forums/index.php/topic,8164.0.html Other than that, you're on the right track by checking e-bay frequently.